The present invention relates to an optical transmission line for wavelength division multiplexed optical signals, and, more particularly, to a wavelength dispersion compensation system for an optical transmission line in an optical wavelength division multiplex transmission repeating system which uses an optical direct amplifier.
In an optical communication system using an optical fiber as the transmission line, wavelength dispersion compensation is performed due to wavelength dispersion characteristics of the optical fiber. Japanese Laid-open Patent Application No. 62-275204 discloses technology of the wavelength dispersion compensation system. A wavelength dispersion compensator (WDC) is provided to compensate for wavelength dispersion on an optical fiber, which is an optical transmission line between a transmitter and a receiver. The wavelength dispersion compensator is arranged to compensate for wavelength dispersion by the optical transmission line so that entire wavelength dispersion from the transmitter to the receiver is made equivalent to zero. The wavelength dispersion compensator makes the entire wavelength dispersion substantially zero by using optical fiber which has a wavelength dispersion value whose polarity is opposite to that of the optical transmission line, and by controlling the length of the optical fiber.
When such conventional wavelength dispersion compensation system is applied to an optical wavelength division multiplex (WDM) transmission line, there arises the following problem. Assume that the multiplexed wavelengths are n wavelengths of .lambda.1-.lambda.n (n being integer of 2 or more), and that n optical signals with different wavelengths are multiplexed, transmitted and repeated over transmission lines. In such system, if the wavelength dispersion characteristics of the dispersion compensator are defined by an optical fiber optics which provides zero dispersion characteristics for an optical signal wavelength of .lambda.n, the wavelength dispersion of the other optical signals having wavelength .lambda.1-.lambda.(n-1) still remains.
The conventional dispersion compensation system compensates for the wavelength dispersion of an optical signal of wavelength .lambda.n so that the dispersion becomes zero. However, this compensation is ineffective for the other optical signals of wavelength .lambda.1-.lambda.(n-1).